Climate scientists have come to the startling realization that — owing to inaccurate and sparse measurements prior to 2004 — we've been seriously underestimating the rate at which the ocean in the Southern Hemisphere is getting hotter. And as scientists like to say, ocean warming is global warming.

By using satellite data and a suite of climate models, scientists from Lawrence Livermore National Laboratory have discovered that long-term ocean warming in the upper 700 meters of the Southern Hemisphere's oceans has likely been grossly underestimated. The team analyzed ocean temperature data collected between 1970 and 2004 and compared the results to climate models.

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The researchers also considered sea surface height, an important indicator of global warming; as oceans warm, sea levels rise. Their models showed that the relative change in sea surface height was consistent with satellite observations taken since 1993. But climate model estimates of warming in the top 700 meters of the world's oceans are not consistent with ocean temperature data extracted before 2004.

"Prior to 2004, research has been very limited by the poor measurement coverage," noted LLNL oceanographer Paul Durack in a statement. "[O]ur results suggest that global ocean warming has been underestimated by 24 to 58 percent. The conclusion that warming has been underestimated agrees with previous studies, however it's the first time that scientists have tried to estimate how much heat we've missed."

So if he's right, the oceans in the south have been absorbing more than twice as much of the heat trapped by our excess greenhouse gases that previously calculated — and that the world is warming faster than we thought.

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Indeed, ocean heat storage is a critical consideration. It accounts for more than 90% of the Earth's excess heat associated with global warming, while the Southern Hemisphere oceans make up 60% of the world's oceans. This study obviously has serious implications for how scientists view the Earth's overall energy budget.

"There has been a general acknowledgement in the literature, that southern-hemisphere estimates of ocean warming are likely biased low," says Durack. "Our study is the first to attempt to quantify the magnitude of what this generally acknowledged underestimate is, using as much information as is available."

The study covers the period from 1970 to 2003. Cai says that, during that time, while the northern hemisphere has been well sampled by cargo ships and projects led by wealthy countries north of the equator, very few direct measurements have been taken in the south. So it's not surprising that the in-situ measurements have been wrong. "But this is huge," says Cai.

"One could say that global warming is ocean warming," Gregory Johnson and John Lyman at the US National Oceanic and Atmospheric Administration wrote in a commentary accompanying Durack's paper. "Quantifying how fast, and where, the ocean is warming is vital to understanding how much and how fast the atmosphere will warm, and seas will rise."

Since 2004, automated profiling floats — called Argo — have been used to measure global ocean temperatures from the surface down to 2,000 meters. The 3,600 Argo floats currently probing the world's oceans are offering unprecedented systematic coverage of the Southern Hemisphere.

Photo by Alicia Navidad/CSIRO.

These results are consistent was a related paper that appears in the same issue of Nature Climate Change. Scientists from NASA's Jet Propulsion Laboratory found that from 2005 to the present, measurements recorded a continuing warming of the upper-ocean. The latest observations showed that this upper-ocean warming and satellite measurements are consistent.